Revivification of a silicious cracking catalyst by treating with fluosilicic acid



June 1954 s. a. SWEETSER I REVIVIFICATION OF A SILICIOUS CRACKING CATALYST BY TREATING WITH FLUOSILICIC ACID Filed Feb. 28, 1951 r 2 Sheets-Sheet 2 Sumner 5. 'Sweetser {Savenbor E35 w Qtoorneg Patented June 15, 1954 REVIVIFICATION OF A SILICIOUS CRACK- ING CATALYST BY TREATING WITH FLUOSILICIC ACID Sumner B. Sweetser, Cranford, N. J assignor to Standard Oil Development Company, a corporation of Delawar Application February 28, 1951,

13 Claims. 1

The present invention relates to the revivifica tion of catalysts used in catalytic hydrocarbon conversions involving hydrocarbon cracking. More particularly, the invention pertains to a process for revivifying catalysts which are subject to contamination and deterioration by the deposition of contaminants resulting from the prolonged use of the catalysts in the continuous type of catalytic hydrocarbon cracking operations wherein a subdivided catalyst is circulated b tween a cracking zone and a regeneration zone, heat required for the cracking reaction being supplied as sensible heat of catalyst reheated in the regeneration zone by the combustion of carbonaceous catalyst deposits, In its broadest aspect, the invention contemplates the reviyification of catalysts deteriorated as the result of their prolonged use in such operations, by a treatment with fluosilioic acid (HzSlFe).

In conventional processes for the catalytic cracking of hydrocarbon oils the feed stock, usually a gas oil, is contacted in the vapor state with a cracking catalyst at temperatures of about 800-1000 F. and pressures ranging from subatmospheric to 50 p. s. i. g. or higher. Activated clays including acid treated bentonite, synthetic composite gel type catalysts, such as composites containing silica, alumina, and/or magnesia, with or without the addition of activating ingredients, such as boria, chromia, etc. have been used.

In the course of the cracking reaction, cokelike carbonaceous materials are deposited on the catalyst. ihese contaminating deposits seriously impair the activity of thecatalyst. The contaminated catalyst is, therefore, regenerated by ing gas such as air at temperatures of about B-1200 F. In the continuous type of operation here involved, the catalyst is circulated con 2inuously between the reaction zone and a carbon- )urning regeneration zone for the purpose of :atalyst regeneration and heat supply.

However, the catalyst also undergoes a second ype of deactivation. It has been found that ,fter a prolonged use the catalyst shows a marked [ecline in activity and particularly in selectivity oward the formation of gasoline range hydroarbons over'ultimate cracking products, such as arbon and gas. This deterioration of the catast must be remedied by procedures other than 1e combustion type of regeneration. The pres- 1t invention refers to such a procedure.

Various methods for reviving catalysts deiriorated in this manner have been proposed Serial No. 213,130

2 heretofore. The most efiicient methods of this type involve a treatment of the catalyst with certain chemicals, particularly fluorine comacid yielding hydrogen fluoride and/or free 11110- rine so as to increase the activity of the fresh catalyst or similarly to treat used catalysts with HzSiFs in order to replace fluorine in catalysts, the activity of which depends on the presence in the fresh catalyst and which have been depleted in fluorine content in the course of the cracking operation.

The utility of all these known processes is directly orindirectly predicated on a corroding or etching 'efi'ect of fluorine or hydrogen fluoride on the surface of the catalyst. If this effect is not closely controlled, the damage done by an excessive removal of active catalyst constituents or by other undesirable changes in the active catalyst centers may nullify or even outweight the beneficial effects of the treatment with fluorine compounds. The careful control thus required greatly complicates these known procedures. The present invention overcomesthis disadvantage.

It is, therefore, the principal object of the present invention to provide improved means for revivifying cracking catalysts which have been deteriorated in prolonged continuous operation involving catalyst circulation. A more specific object of the invention is an improved method for revivifying such catalysts without corroding or etching effects on the catalyst. Other objects and advantages will appear from the subsequent description of the invention wherein reference will be made to the It has now'been found that the gasoline selectivity of cracking catalysts, more particularly synthetic mixed gels of the silica-alumina type which are deteriorated with respect to gasoline selectivity through prolonged use in a system involving continuous catalyst circulation, may be substantially increased by incorporating into the used catalyst about 0.5-5 weight percent, preferably about 1-2 weight percent, of fluosilicic acid at conditions not conducive to the liberation of HF or free fluorine from the acid. The treatment in accordance with the invention may be carried out by soaking the suitably subdivided catalyst in an aqueous solution of HzSiFe of about 240% concentration at temperatures of about 150 F. for a time sufiicient to incorporate by conditions of the cracking process. The result by the treating agent is that catalyst corrosion is avoided in all stages of the process and that losses of treating agent in the course or the revivification and cracking process are insignificant. The revivifying effect secured in accordance with the present invention is, therefore, much more persistent than that of fluorine compoundsused in accordance with prior art procedures.

While the invention aiiords connection with most conventional lysts, best results are consistently synthetic composite cracking cataobtained when catalyst best suited for the purposes of the invention is a composite of this type. containing about 85-88%.silica gel and about 12-15%, preferably about 137%, of alumina.

The process of the invention has utility'in all improvements in more clearly hereinafter.

ing 13% concentrationbe used at temperatures gels comprising silica gel andzw Y V minor prgpgyfigns of amminam-e employed The corporation of haSiFs in order to prevent decorne 1'.

position and resulting corrosive action of the acid... 1 iiThe beneficial effects of the process of the in.

elude temperatures of about 700-1100 F. and atmospheric or slightly elevated pressure. Catalyst fouled by coke deposits is withdrawn from the fluidized bed, suspended in air and regenerated by carbon combustion in a regenerator in the form of a similar fluidized solids mass at higher temperatures of about 1000-1200F. Hot

regenerated catalyst is suspended in th oil feed and. returned to the cracking zone.

When employing the invention in combination with ajsystem of thistype, catalyst selectivity may be maintained ata desirable level by withdrawing catalyst at the rate of about 110% of catalyst inventory per day, incorporating about OLE-5% of HzSiFe into the withdrawn catalyst and returning the catalyst so treated to the system. This-treatment may be carried out in continuous or batch operation as will appear It is essential, however, that dilute solutions of HzSiFe not exceednot substantially exceeding 150 F. for the in- 5 vention are illustrated by th experimental data reported below. A alumina cracking catalyst containing about 13% typesofcontinuous cracking operations involving catalyst circulation between reaction and regeneration zones; such as fluid, movingbed or. suspensoid system s.-, These operations differ basically: from fixed bed operationswith respectto catalyst deterioration; Catalyst deactivation by carbon deposition occurs in fixed bed aswell as in continuous operation. However, the type of;

catalyst deterioration which recuiresreviviiying treatments in addition to the conventional re-.

generation by carbon removal presents a serious problem only in continuous operation. The catae' lyst contaminants responsible: for this type-oi catalyst deterioration are partly introduced with the oil feed, particularly in th form of metal catalystin the form-cf iron I In fixed: bed operation the feed contaminants are retained by the catalyst layers v equilibrium activity conventional gel-type silica of alumina, having an average particle size of about 50-80 microns and having reached. an

catalytic cracking plant comprisinga cracking zone operated at about 925 F, and a combustion type regeneration zone operated at about 1100 of carbon was taken Rrwas used. A sample of this lyst containing less than about 1 weight per cent from the circulating catalyst stream.

One portion of the catalyst was wettedat room temperature with about 28 c. c. per 100 grams of catalyst, of an aqueous'fluosilicic acid solution 3 containing about 5% of HzSiFs. rial which had the appearance of wet sand was salts, and partly picked up by the circulating thoroughly mixed for about 5 minutes and then from equipment walls.

by the feed, which act as a filter bed and no iron tionary catalyst. In continuous operation, however, th feed: contaminantsare deposited on the, entire catalyst and an eroding effect on the equipment walls. resultis picked-up irom'equipment walls-by the sta--' the circulating catalyst has standards of ing in excessive catalyst contamination. These-- effects are particularly pronounced in the fluid catalyst technique which involves circulation oflarge amounts of catalyst at relatively high velocities. On the other hand,

handling, heat transferand completeness oiygasits greatest utility ferred embodiment of the invention.v

In conventional fluid operation, powdered catalyst having an average. particle size ofabout;

this technique afford-s greatest advantages with respect to solids, Y Briefly 1 equipment.

50-80 microns is contacted with the vaporized feed inthe form of a dense-turbulent fluidized solids mass'simulating a boiling liquid in appearance and hydrostatic and hydrodynamicproperties-.- Fluidization isaccomplished by passdried in The wet matean oven at 2l0-225 F. in an atmosphere of air. -The dry catalyst contained 1.4 weight fir t contacted A per cent of l-lzSiFs. The portion of the catalyst sample so treated and the untreated portion were pilled and testedin a fixed bed standard laboratorytesting unit to determine. their activity and selectivity.

The well established activity and selectivity Distillate-l-Loss (D+L) Carbon Producing Factor, (CPF), and .fGas Producing [g Factor (GPF) were rolled on. The D+L of a catalyst is determined by a standard test fully described in the literature Ind, Eng. Chem. .vol. "39, page 1138 (1947).). a standardised stock is, cracked on the test catalyst atstandard conditionsand 100 c. c. of the liquid productis The amount. of gasoline distilled at 400 F. is designated as Distillate? or D, the residue is R and the value lOO-(D-l-R) is called loss orL. The sum of D+L is a criterion for catalyst activity and may vary from about 10-20 for deactivated catalysts to about 25-50 for re activated or fresh crackingcatalysts. The Car- 7 icon Producing Factor (CPF, is the ratio of the ing the 'feed vaporsupwardly through the solids" bed at velocities of about 0.3-3 ft. per second'to establish apparent bed densities of about 10-30 I lbs. per cu. ft. Suitable cracking conditions'incarbon produced by the test catalyst to that pro duced by an uncontaminated catalyst adjusted to the same activity by steam or'heattreatment.

. It ranges from 1.0"lor fresh catalyst to'1.5- .0

or more" for used catalyst; The ratio of dry gas (cu. ft. per barrel of oil) producedby the test in a commercial fluid-type equilibrium? cata- (Conn and Connolly,

distilled in standard V nated catalyst of Producing Factor fluid catalytic cracking size of about 40-89 microns.

produced by uncontamithe same activity is the Gas (GPF). Both CPF and GPF' may be used to measure catalyst selectivity which is the better the lower these factors.

The results obtained in so testing the treated and untreated catalyst portions described above were as follows.

catalyst to the dry gas Gas C, per- Catalyst fi fii wt, cent on CPF V GPF percent feed Untreated 23.0 6. 2 1.8 1. 96 2. 03 Impregnated with 1.4%

HaSiFa 28. 5. 0 1. 2 1. 00 1. 45

The above data show that catalyst activity as determined by D+L as well as catalyst selectivity as determined by CPF and GPF were significantly improved upon treatment of the catalyst in accordance with the invention.

As pointed out above, the invention finds its greatest utility in connection with fluid catalytic cracking. Two modifications of this preferred embodiment of the invention are illustrated in th accompanying drawing wherein Figure 1 is a schematical flow plan of a system adapted to carry out the process of the invention in a continuous manner in combination with a unit, and

Figure 2 is a simplified illustration of a system employing batch operation in th catalyst treating section.

Referring now to Figure 1 of the drawing, the system illustrated essentially comprises a fluidtype cracking reactor ill, a regenerator 25 of similar design, a mixer 49, a reslurrying zone 50 and a spray drier 55. The function and coaction of these elements will be forthwith described.

In operation, a gas oil boiling within the range and preheated to a temperature of about 500-800 F. may be supplied to, the system via line i. The oil feed is discharged into line 3 wherein it is mixed with hot regenerated subdivided catalyst, such as a synthetic silica-alumina mixed gel, supplied to line 3 from line 5 at a temperature of about 1000 1100 F. The catalyst-oil mixture containing about 3-10 lbs. of catalyst per lb. of oil is introduced at a suitable cracking temperature of about 900-1000 F. from line 3 through a distributing device such as grid 1 into reactor III to form therein above grid 1 a relatively dense turbulent mass of solids M fluidized bythe reacting hydrocarbon vapors and, if desired, by additional steam added via line 9. Reactor I 0 may be designed for a superficial linear gas velocity of about 0.54.5 ft. per second and an apparent density of mass M10 of about 20-40 lbs. per cu. ft. assuming an average catalyst particle Cracked oil vapors and entrained catalyst fines leave reactor 10 through gas-solids separation means such as oyclone separator Ii. Separated solids may be returned to mass M10 via dip-pipe I3 and product vapors may be passed via line [-5 to conventional product recovery equipment (not shown).

Catalyst contaminated with carbonaceous deposits may be withdrawn from mass M10 via a conventional aerated standpipe i1 substantially at the rate at which catalyst is supplied to reactor it via line 3. The withdrawn catalyst, preferably after conventional stripping, is discharged into line 89 wherein the catalyst is suspended in air supplied from line 2!. Th suspension is passed via grid 23 to regenerator 25 the catalyst in mass M25 to form therein a fluidized mass M2 similar to mass M10. carbonaceou deposits are burned ed at temperatures of about 1000-l200 F. Flue gases leave regenerator 25 through cyclone 2! and line 29, separated solids being returned to mass M25 via dip-pipe i l. Fresh make-up catalyst may be added via line 33 as required. Hot regenerated catalyst is withdrawn from mass M25 via conventional aerated standpipe 5 and, preferably after conventional stripping, returned to l ne 3 as previously described. The process described up to this point is conventional, various procedural and equipment details obvious to the expert having been omitted for the sake of simplicity.

After the system has been operated for a certain length of time in the manner described, the selectivity of the catalyst has dropped from a CPF value of l for the fresh catalyst to a CPF value approaching or exceeding 2 causing the operating efficiency to drop sharply. At this or any desired earlier time the treatment in acccrdance with the invention is employed. For this purpose, a catalyst stream amount to about 140% of the total catalyst inventory of thesystern per day may be branched on standpipe. 5 and passed through line 35 to a cooler 3'! where in the catalyst temperature is reduced to a tem perature of about -150 F.

The cooled catalyst flows through line 39 to a mixer All provided with kneading arms M or the like. An aqueous solution of fluosilicic acid of about 12-10% concentration is sprayed from line .3 at room temperature onto the catalystin about (lb-5% of HzSiFs to the catalyst. About 0.06-0.085 gal. of acid solution per lb. of catalyst is sufficient for this purpose. The catalyst and acid are intimately mixed for a period of about 2-10 minutes and then moved through mixer 48 by kneaders M. The mixture is picked up by a screw conveyor 25 and passed to reslurrying vessel 47 provided with stirrer 43. Water suflicient in amount to convert the mixture into a punipable slurry is supplied to vessel 41 from line 49. The catalyst slurry formed is spray drier 55 supplied through line 5i with hot air having a temperature of about 225-800 F. Waste air is withdrawn through a cyclone separator 59 and line 6!. Dry reactivated catalyst is withdrawn from drier 55 via a conventional aerated standpipe 63 and may be supplied to oil feed line 3 and reactor Hi substantially at the rate of catalyst withdrawal via line 35.

The system of Figure 1 permit sof various modifications. For example, in place of passing the impregnated catalyst mixture to a reslurrying zone and a spray-drying zone the impregnated catalyst may be passed to a rotary kiln type drier. In this case it may be desirable to granulate the dried catalyst in order to break up loose agglomerates formed in the drying operation. Other modifications may appear to those skilled in the art.

The system described with reference to Figure 1 of the drawing is particularly adapted for continuous operation of the reactivation equipment and, therefore, highly useful in combination with commercial cracking plants of relatively large size. When the smaller size of the cracking plant permits batch reactivation of the catalyst or such a procedure is desirable for other reasons,

sasystem of: the typeillustrated .i;n;Figure 2 of The rate, of charging and discharging to and :the drawing may be used. from the-reslurrying vessel is regulated to main- Referring now toFigure 2 of the drawing, tain" 63 gallons of slurry thereinand thereby to .thefdesign and operation of the cracking and give a 2 minute residence time of the. slurry in .;combustion regeneration stages are essentially 5 this zone. The reslurried catalyst is passed ,the same as those described with reference to .:through slurry pump 53 into spray drier'55 where Figurel, like reference characters identifying like it is contacted with, hot air or regeneration gas :equipment elements. in order to obtain dry catalyst. The amount When the selectivity 'of the catalyst has of hot gas required for the drymg operation is :dropped to a point requiring revivifieation, spent about 2 million cu. 'ft. per hour at a temperature 1 catalyst is withdrawn through line 35 and cooler of about 725 F. The catalyst is discharged from .31 and supplied at approximately room temthe drier at a temperature of .225 F. The disrperatule to Storage pp r until bou 8% charged catalyst is continuously mixed with the of 'spent catalyst inventory is collected in hopper stream of catalyst circulating in line 3 from the may be stopped and about 10-20% of this cata- 10. Thereafter catalyst flow through line 35"' regenerator to the reactor.

' :lyst may be supplied by means of screw conveyor Example H I '12 to mixer it. About 0.06-0.085 gal. of 1-5% .In a fl i c t ly racking un t h v n a f d ":aqueous fiuosilicic acid per lb. of catalyst is supp y of about 5,000 barrels 1 y and With :plied to mixer 12 via line 16. The acid and solids 30 a 60 ton inventory of equilibrium silica-alumina 1are:mixed in mixer 12 by means or knead-ere cracking catalyst, a system of the type oigF-igure r18 for about 520 minutes. The pasty mixture 2 may be employed as follows. The-amount of .is transferred to a drier 82 which may be a tray catalyst to be treated per day is 6 tons. Two

.drieror of any other conventional design. Hot tons of catalyst. is withdrawn from regenerator "air may be introduced through line 84 at a tem- "Z5 :25, cooled to. a temperature of about 250 F. in

perature of about 200-225 F. for about 26 cooler 37 and discharged into hopper it. A 500 hours. The dried catalyst which is now .in the lb. batch of catalyst is transferred from. hopper :form of lump-sized agglomerates may be passed 10 into mixer it while itis sprayed with 38 galto a conventional granulator or grinder 38 to be lons of a 2.4% solution of fiuosilicic acid which converted to particles of fiuidizable size, e. g. is at room temperature. The catalyst and soluto a powder passing 100 mesh. This powder may tion are mixed in the vessel for about 10 minbe supplied to a feed hopper st. Hopper 90 may ute s. The impregnated catalyst is transferred be aerated with small amounts of an inert gas, on to trays holding about 100 lbs. of catalyst.

' such as nitrogen, flue gas, etc. to maintain the The trays are placed in a steam oven 82 where catalyst therein in a readily flowing c0ndition.'ii5 they are maintained for a period of .4 hours at Revivified catalyst ready for reuse may be rea temperature of 225 F. The dried catalyst is -turned via conventional standpipe 9t and oil i'eed dumped into granular 38 containing a 40 mesh line 3 to the cracking system as needed to im- 1 screen where it is treated to-breakup loose prove the selectivity of the catalyst in reactor agglcmerates which are formed in the drying H), e. g. at approximately the rate at which -10 operation. Granulated catalyst is transferred catalyst was withdrawn through line 35. When into hopper 99. The above operation is then the catalyst supply in hopper St is exhausted, repeated with another 500 lb. batch of catalyst the revivification treatment may be repeated with until the complete contents of the hopper ii] 'a fresh batch of spent catalyst withdrawn have been treated. At suitable intervals the revivthrough line 35. ified catalyst in hopper $0 is discharged into The system of Figure 2 may be modified in the catalyst stream in line 3 circulating between various respects without deviation from the spirit l the regenerator and the reactor.

- trays.

of the invention- For example, arotary kiln type If the operation is spread over the day on a drier maybe used in place of oven drying on 3 continuous basis, a drying oven 82 holding 20 trays will have suficient capacity for treating The Process of h l t flm Will b urth the catalyst. On the other hand, if the work is illustrated by the following specific examples. 7 to be completed in an 3 hour day, a drying oven 7 of 3-4 times this capacity will be required. Example I g The ioregoing'description and exemplary op- A system of t type mustmted in Figure Aerations have served to illustrate specific emmay b employed s follows In a fluid Catalyst 7 bodiments of the invention but are not intended cracking plant havinga capacity of about 40,000 tobe limiting n sc p barrels per day of feed and having a catalyst What is claimed is: inventory of 450 tons of i an equilibrium silica 1. In the method of reviviiying spent siliceous alumina cracking catalyst, the catalyst is with-+ cracking catalysts which have been contaminated drawn through line 35 at the rate of 1.41 tons with metallic type contaminants and the gasoline .per hour, cooled in cooler 31 to a temperature selectivity of which. has been reduced by extended of about 250 F. and then discharged into mixer use in a hydrocarbon cracking system wherein 40. A. solution containing 1.5% by weight of the catalyst is continuously circulated between a .fiuosilicic acid at a temperature of F. 1s 5: cracking zone and a combustion-type regenerapumped into the mixer at a rate of 214 gallons per 7 tion zone, the improvement which comprises withhour. The temperature of the mixture of cata- :drawing a portion or" said catalyst of reduced lyst and impregnating solution is about F. selectivity from said system, incorporating into "The rate of charging and discharging of catalyst said. withdrawn catalyst about 0.5 to 5% by weight to and from the mixer is controlled so that the 70 of fiuosilicic acid at conditionsoi temperature :mixer contains 4'70 lbs. of catalyst at all times within the range or 75 to F. non-conducive and the catalyst, therefore, has a contact time to the decomposition of said acid to .inacti "therein of about 10 m n t se mp g vate contaminants, drying and granulating the catalyst is dischar d into reslurrying v ssel 41 catalyst, and returning catalyst so treated to said andmixed with 1,880 gallonsv of water per hourz fii system whereby said contaminants are. substantially rendered incapable of deactivating said catalyst.

2. The process of claim 1 in which the catalyst is maintained in said cracking and regeneration zones in the form of dense turbulent solids masses fluidized by upwardly flowing gases to resemble a boiling liquid.

3. The method of revivifying spent siliceous cracking catalysts the gasoline selectivity of which has been reduced by extended use in the cracking of hydrocarbon oils due to metallic type contaminants, which comprises incorporating into spent catalyst about 0.5 to by of fluosilicic acid at a temperature within the range or about 75 to 150 F. and non-conducive to the decomposition of acid to deactivate contaminating materials and thereby restore said catalyst to activity.

4. The method of revivifying spent siliceous cracking catalysts the gasoline selectivity of which has been reduced by reason of accumulated metallic type contaminants by extended use in the continuous catalytic cracking of hydrocarbon oils to such a degree that it can not be restored by combustion-type regeneration, which comprises impregnating said spent catalyst with an aqueous solution of fluosilicic acid or not more than 13% acid concentration at a temperature within the range of about 75 to 150 F. and non-conducive to the decomposition of said acid until about 0.5-5 weight percent of said acid is incorporated into said catalyst and drying the impregnated catalyst whereby the said contaminants are substantially rendered incapable of deactivating the catalyst.

5. The method of claim 4 in which said catalyst is a silica-alumina composite containing about 12-15% of alumina, said aqueous solution having an acid concentration of about 210% and said temperature being about 75-150 F.

6. The method of claim 5 in which about 1-2 weight percent of fluosilicic acid is incorporated into said catalyst.

'7. In the method of revivifying spent silicaalumina cracking catalysts the gasoline selectivity of which has been reduced due to accumulation of metallic type deposits resulting from extended use in a fluid-type hydrocarbon cracking system wherein the catalyst is continuously circulated between a fluid-type cracking zone and a fluid-type carbon-burning regeneration zone until the gasoline selectivity of said catalyst has been reduced to such a degree that it can not be re stored by regeneration in said regeneration zone, the improvement which comprises withdrawing from said system a minor proportion of the total catalyst inventory of said system, impregnating said withdrawn catalyst with an aqueous fiuosilicic acid solution of not more than 13% acid concentration at a temperature within the range of about to and non-oonducive to the decomposition of said acid until about 0.55 weight percent of said acid is incorporated into said catalyst, drying said impregnated catalyst and returning said dried catalyst to said system substantially at the rate at which said catalyst was withdrawn from said system whereby said deposits of metallic type are rendered incapable of deactivating the catalyst.

8. The process of claim 7 in which said catalyst is continuously withdrawn from said system, continuously impregnated and dried, and said dried catalyst is continuously returned to said system.

9. The process of claim 8 in which said impregnated catalyst is slurried in water to form a pumpable slurry and said slurry is subjected to spray drying to form said dried catalyst.

10. The process of claim 8 in which said impregnated catalyst is dried while being subjected to a tumbling motion and said dried catalyst is granulated to a fluidizable size prior to being returned to said system.

11. The process of claim 7 in which said catalyst is withdrawn from said system in intervals and said dried catalyst is returned to said system in intervals, said impregnating and drying being carried out in batches.

12. The process of claim 10 in which said dried catalyst is granulated to a fiuidizable size prior to being returned to said system.

13. The process of claim 7 in which said minor proportion is about 1-10%.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN THE METHOD OF REVIFIFYING SPENT SILICEOUS CRACKING CATALYST WHICH HAVE BEEN CONTAMINATED WITH METALLIC TYPE CONTAMINANTS AND THE GASOLINE SELECTIVITY OF WHICH HAS BEEN REDUCED BY EXTENDED USE IN A HYDROCARBON CRACKING SYSTEM WHEREIN THE CATALYST IS CONTINUOUSLY CIRCULATED BETWEEN A CRACKING ZONE AND A COMBUSTION-TYPE REGENERATION ZONE, THE IMPROVEMENT WHICH COMPRISES WITHDRAWING A PROTION OF SAID CATALYST OF REDUCED SELECTIVITY FROM SAID SYSTEM, INCORPORATING INTO SAID WITHDRAWN CATALYST ABOUT 0.5 TO 5% BY WEIGHT OF FLUOSILICIC ACID AT CONDITIONS OF TEMPERATURE WITHIN THE RANGE OF 75* TO 150* F. NON-CONDUCIVE TO THE DECOMPOSITION OF SAID ACID TO INACTIVATE CONTAMINANTS, DRYING AND GRANULATING THE CATALYST, AND RETURNING CATALYST SO TREATED TO SAID SYSTEM WHEREBY SAID CONTAMINANTS ARE SUBSTANTIALLY RENDERED INCAPABLE OF DEACTIVATING SAID CATALYST. 